No matter how well the background is rendered, inserting the actors in the digital environment isn't as easy as just dropping them in. Even a subtle mistake can throw off things enough to make the scene look awkward. I have a book done by a GREAT digital artist. It is almost impossible to tell his work is rendered. In this book, he has race cars and real people interacting with them. In 1 picture, a guy is standing next to a car with his hand on top. The picture bugged me for a while because there was something off with it. A friend pointed out that , where the guy has his hand, there was a side mirror, and ideally, there should be a shadow on the guys hand. Even that little mistake made the picture look off.
Putting people in real environments helps to reduce that "dropped in" effect when green/blue screen is done poorly.